Computers - EdexcelCommon CPU components

The central processing unit (CPU) consists of a number of components:

• control unit (CU)
• arithmetic logic unit (ALU)
• registers
• cache
• buses
• clock

All components work together to allow processing and system control.

Control unit

The control unit provides several functions. It:

• fetches, decodes and manages the execution of the instructions
• issues control signals that control hardware
• moves data around the system

Arithmetic logic unit

The ALU performs arithmetic and logical operations or decisions. It is where calculations are done and where decisions are made based on logic.

Registers

Registers are small amounts of high speed memory contained within the CPU. They are used by the processor to store small amounts of data that are needed during processing, such as:

• the address of the next instruction to be executed
• the current instruction being decoded
• the results of calculations

Different processors have different numbers of registers for different purposes, but most have some, or all, of the following:

• program counter
• memory address register (MAR)
• memory data register (MDR)
• current instruction register (CIR)
• accumulator (ACC)

Cache

Cache is a small amount of high speed memory built directly within the processor. It is used to temporarily hold data and instructions that the processor is likely to reuse. This allows for faster processing as the processor does not have to wait for the data and instructions to be fetched from the random-access memory (RAM).

Clock

The CPU contains a clock which is used to coordinate all of the computer's components. The clock sends out a regular electrical pulse which synchronises - keeps in time - all the components.

The frequency of the pulses is known as the clock speed. Clock speed is measured in hertz. The higher the clock speed, the greater the number of instructions that can be performed in any given moment of time.

In the 1980s, processors commonly ran at a rate of between 3 megahertz (MHz) to 5 MHz, which is 3 million to 5 million pulses or cycles per second. Today, processors commonly run at a rate of 3 gigahertz (GHz) to 5 GHz, which is 3 billion pulses or cycles per second.

Buses

A bus is a high speed internal connection. Buses are used to send control signals and data between the processor and other components.

Three types of bus are used:

• Address bus - carries memory addresses from the processor to other components such as RAM and input/output devices.
• Data bus - carries the actual data between the processor and other components.
• Control bus - carries control signals from the processor to other components.